The present invention is described below with respect to its application to the manufacture of exterior automotive body panels, although other end-uses of the films made by this invention also are considered to be within the scope of this invention.
Exterior automotive body panels have been made in the past by spray painting sheet metal parts. Multi-layer paint coats, such as those referred to as a clear coat/color coat paint finish, have been used to produce desirable optical effects. In addition to high gloss and high distinctness-of-image (DOI), these paint coats also are highly durable by providing chemical resistance, abrasion resistance and weatherability that significantly reduces degradation by ultraviolet light.
In more recent years molded plastic car body panels have been made with decorative clear coat/color coat paint films bonded to the molded plastic panel. Use of such films avoids certain environmental problems associated with evaporation of paint solvents while also reducing or eliminating the need for paint facilities and emission controls at the automotive production plant.
Because of the growing need to reduce the amount of atmospheric pollution caused by solvents emitted during the painting process, many different approaches have been taken in recent years for producing these decorative films. These processes are generally categorized by solution casting techniques or extrusion techniques. For instance, U.S. Pat. Nos. 4,810,540 to Ellison et al. and 4,902,557 to Rohrbacher use solution casting techniques in which liquid-cast, solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating or gravure printing. The liquid cast layers are separately applied and then dried at high temperatures to evaporate the solvents.
As an alternative, extruded films have been used for making exterior automotive clear coat/color coat films. International Application PCT US93 07097 to Duhme describes a process in which an injection molded laminate is made from an extruded clear coat layer, a color coat layer, a reinforcing layer laminated to the color coat layer, a bonding layer on a side of the reinforcing layer opposite from the color coat, and an injection molded substrate bonded to the bonding layer. The outer clear coat layer is a coextruded sheet having different proportions of polyvinylidene fluoride (PVDF) and acrylic resins in each layer of the coextrusion. An extruded thermoplastic liner layer is laminated to the outer surface of the clear coat layer to assist in injection molding the paint film laminate to the substrate. The coextruded outer clear coat layer is laminated to a polyester carrier which supports the clear coat layer during subsequent lamination steps. The outer clear coat layer can optionally be extruded onto the thermoplastic liner layer to provide gloss control. The color coat is made by solvent casting it on a carrier and laminating the dried paint coat to the clear coat. The reinforcing layer is laminated to the exposed side of the color coat, and the bonding layer may be coated on or laminated to the reinforcing layer. This process involves time-consuming multiple coating and lamination steps and slow processing speeds disclosed in the various examples.
U.S. Pat. Nos. 4,317,860 and 4,364,886 to Strassel also disclose coextrusion of multi-layer films such as a two-layer coextrusion of predominantly PVDF on one side and a predominantly acrylic resin on the other side of the coextruded sheet. These unitary structures are used to make molded articles, or to adhere the sheets to a molded polymer.
Film extrusion techniques also have been used in the past for making free films in which the extruded polymeric material is coated on a polished drum. These films are then undercoated with various color coats. The exterior surface of the extruded free film that contacts the drum (and is separated from the drum as a free film) does not have a high gloss and high distinctness-of-image. Also films manufactured in this manner do not have a carrier sheet attached, which makes them hard to handle and easily damaged in subsequent processing.
Another process disclosed in U.S. Pat. No. 5,114,789 to Reafler comprises a pigmented base coat which is solvent-die extrusion coated onto a flexible, stretchable carrier sheet and dried at elevated temperatures to evaporate the solvents, followed by extrusion coating a reactive clear coat on the base coat. The carrier film and extrusion coated paint layers are then heat softened as a unitary sheet and applied to a molded shaped substrate by a shrink wrap process.
In a currently used process for making exterior automotive paint films, a clear coat and color coat comprising blends of PVDF and acrylic resins are cast by reverse roll coater, either by solution or dispersion casting. The film thickness of the paint coats used in the process generally is dictated by end user requirements. In some instances the need to produce relatively thick films can impose certain production constraints. To adequately dry the material and to prevent air entrapment, line speeds are typically at 25 feet per minute. This slow throughput limits the coating capacity of the reverse roll coater and also releases a large amount of organic solvents. This solvent release is particularly evident when a solution-cast PVDF/acrylic clear coat is coated from a solvent-based solution having a relatively high amount of solvent. VOC emissions are high. PVDF has limited solubility and requires strong solvents to dissolve. One such solvent known as N-methyl pyrrolidone (trade name M-Pyrol) is either needed to solubilize the resin in solution casting or used as a coalescing aid in dispersion casting. In addition, cross contamination can occur from solubilizing residual material in previously used drums, hoses, pans, pumps, etc. Also, during coating, the strong solvent can dissolve caked-on resins in a drying oven, causing them to cascade down on the web being coated. As a further concern, these strong solvents are expensive.
Thus, there is a need for producing decorative and protective surfacing films while avoiding the adverse effects of low production line speed, high VOC, cross-contamination, and the use of expensive solvents.
Extrusion techniques can be an alternative that avoids the use of strong solvents and their related solvent emission problems. Extrusion techniques such as those described above, however, have not been successfully adapted to producing high optical quality films at high line speeds and at low cost.
Application Ser. No. 08/793,836 to Enlow et al. describes a solventless extrusion coating process that provides an alternative to both solvent casting and conventional extrusion of polymeric films. Use of the extrusion coating techniques of that invention provide the advantages of avoiding expensive solvents, producing no VOC emissions, and avoiding cross-contamination associated with solvent casting. In addition, the invention has the added advantages of greatly increasing line speed, eliminating steps in the manufacturing process, and reducing the cost of producing clear coat/color coat films. The invention has particular applicability to the manufacture of molded plastic exterior automotive body panels and parts, in that it provides a means for producing extruded high gloss, high DOI (distinctness-of-image) clear coat films of exterior automotive quality.
It has been recognized that solventless extrusion of polymeric materials into highly transparent, essentially defect-free thin film layers is extremely difficult. When such films are extruded for the purpose of providing a high gloss protective outer clear coat layer for an automotive laminate, for example, the layer is typically extruded as a thin film approximately one mil to three mils thick. However, the human eye catches the slightest defects in such a thin outer clear coat layer of high gloss and high DOI when compared with thicker films extruded as sheets or films that do not have the requirements of high gloss and high DOI.
It has also been recognized that even when a high gloss outer clear coat film is extruded as an essentially defect-free film, the film itself can replicate defects present in an underlying laminate to which it is bonded. For example, in an automotive laminate having an extruded polymeric backing sheet and size coat layer, defects can be telegraphed to the surface of a thin protective outer clear coat layer of high gloss. In this instance, defects as small in size as 10 microns or less in the extruded sub-layers can appear as noticeable defects in the high gloss outer clear coat layer.
Generally speaking, polymeric films which are solvent cast are more easily produced as defect-free clear coat films of high gloss and high DOI when compared with films made by solventless extrusion of polymeric materials. The difficulty arises when extruding engineering plastics as high gloss, high DOI clear coat films. The extrusion process by its nature generates defects in the extruded material and there are several sources of these defects, all of which must be addressed in order to ensure the optical clarity and smoothness of the finished extruded film. For example, application Ser. No. 08/793,836 to Enlow et al. describes how high shear and heat generation in an extruded material can cause induced haze and gel formation and resultant optical defects or reduced optical clarity in the extruded film. That publication also describes how reducing heat histories (minimizing heat rise) when compounding PVDF, acrylic and UV stabilizer starting materials can improve the quality of films made from those materials. Modifications to the extrusion process in order to avoid such problems, however, should not adversely affect subsequent thermoforming operations or unreasonably reduce line speed during the production process.
The formulation of the starting material also can affect optical clarity. For instance, an optically clear film made from a blend of PVDF and acrylic resins can be extruded more haze free when the PVDF component of the starting material is reduced from a level of 70% to below about 65%.
Although the effects of gel formation and induced haze are minimized by the processing techniques described above, it has been discovered that use of these processing controls may not categorically produce extruded clear films of extremely high transparency free of defects because additional defects can be introduced from other sources.
The present invention is based in part on a recognition that film quality of a solventless extruded clear film can be adversely affected by airborne particulate substances that may enter the extrusion process from a variety of sources. Failure to remove these contaminants from the process can result in noticeable defects in a thin extruded high gloss clear film. These defects can adversely affect the finished product whether they are present in the extruded outer clear coat film or in an underlying size coat and/or substrate panel to which the protective clear film is bonded.
It has been discovered that micron-size airborne contaminants from various sources can pass through the extrusion process and end up creating optical defects in the finished product. For, instance, dust particles 10 microns in diameter or less produce noticeable defects in an extruded transparent one mil thick high gloss film. Such defects from airborne contaminants also may not appear until the finished laminate is thermoformed which can cause the defects to appear at the surface. Such airborne contaminants can include not only dirt particles from the air but also fiberglass particles and polymer dust present in the production plant. These contaminants can be introduced into the extrusion process when the resinous starting materials are handled before or after film extrusion.
In addition, contaminants may be present in the resinous starting materials. Such contaminants may include glass fibers, carbon, metal bits and gels introduced from the resin manufacturing process.
Thus, a process for solventless extrusion of thin high gloss clear coat films must address the problems of: (1) avoiding gel formation and induced haze; (2) avoiding defects being introduced not only in an extruded outer clear coat film but also in underlying extruded substrate layers; (3) avoiding film handling problems while maintaining high production line speed; (4) avoiding introduction of contaminants from the starting materials and during the resin handling and extrusion process; and (5) providing a finished laminate that maintains high gloss and high DOI after the finished part is subjected to thermoforming temperatures and resultant elongation.
Although the invention is described above with respect to exterior automotive applications, the invention also has applicability as a protective and decorative coating for other articles such as interior automotive components, exterior siding panels and related outdoor construction products, marine products, signage, window glass and other interior or exterior film products. Vinyl (PVC) siding panels are an example of one use of the invention for producing outdoor weatherable decorative surfaces on extruded plastic sheets. The invention, however, is applicable to plastic substrate panels other than vinyl, such as polycarbonate, for example. The invention is particularly applicable to protective films having a requirement of high transparency free of optical defects, i.e., any protective film that would have glass-like optical properties.